It is an increasingly common requirement in communications to have increased flexibility and interchangeability between handsets and/or headsets and the telephone terminals to which they connect, i.e. to be able to connect an earphone/microphone combination or headset unit to a different telephone terminal. To ensure that a signal with a suitable amplitude is transmitted from the headset via the telephone terminal to the destination (which could be another telephone unit), the necessary degree of amplification or attenuation between the telephone terminal and headset needs to be determined. If the transmitted signal has too low an amplitude then it will be too weak to hear easily at the destination. However, if the transmitted signal has too high an amplitude then it will be distorted at the destination. The signal amplitude from the telephone terminal must be optimized so that the signal sent to the headset can be heard clearly at the destination without clipping or distortion of the signal due to overdriving of the terminal. Clipping occurs due to the saturation of the microphone input circuit to the telephone terminal amplifier.